1. Field of the Invention
The present invention relates to a ferrite composition preferable for manufacturing such as a multilayer inductor, and to an electronic device having a ferrite sintered body composed of said composition.
2. Description of the Related Art
Recently, down-sizing and high frequency of DC-DC converters are progressed. And the DC-DC converters driven by around several tenths MHz to several hundreds MHz frequency are appeared. For inductors applied to the DC-DC converters which down-sizing and high frequency are progressed, frequency characteristics, in which the inductors are driven even in a high frequency region, and DC superposition characteristics, in which the operation does not substantially change even when large current is applied, are required.
As ferrite compositions used for the inductors applied to the DC-DC converters, NiCuZn ferrite added with cobalt is being proposed from before. By adding cobalt to NiCuZn ferrite, methods to improve magnetic anisotropy and to improve frequency characteristics of permeability have been examined.
It is known from before, however, that sintering property tends to lower and temperature characteristic tends to deteriorate the NiCuZn ferrite added with cobalt, relative to the NiCuZn ferrite without an addition of cobalt. The methods shown below are proposed, in order to overcome the demerit of the above-mentioned cobalt addition.
Patent Article 1 discloses NiCuZn ferrite which overcomes the demerit of sintering property by adding bismuth compound, such as bismuth oxide, together with cobalt compound. In addition, Patent Article 1 discloses NiCuZn ferrite, which is trying to improve the temperature characteristics by adding zirconium compounds.
Patent Article 2 discloses NiCuZn ferrite in which Co3O4, SiO2 and Bi2O3 are added, trying to obtain a high Q value, a superior temperature characteristic, and a high proof stress.
Patent Article 3 discloses NiCuZn ferrite in which amounts of Fe2O3 and ZnO are relatively small, and CoO is further added, trying to obtain a ferrite composition superior in frequency characteristic.
Examples of Patent Article 1 shows initial permeability μi value, however, how high frequency this permeability value is maintained is not described. According to Snoek's limit, in general, the permeability value is maintained to a high frequency as the value of the initial permeability μi is low. However, in case of adding additives, the permeability is maintained to a high frequency to Snoek's limit or more; and to the contrary, the permeability lowers to a low frequency to Snoek's limit or less. Thus, the initial permeability μi may become an example of the frequency characteristic, however, there is little evidence to become an evaluation criteria of the frequency characteristic. Thus, the frequency characteristic is unknown in the examples of Patent Article 1.
Note Snoek's limit is shown by the following equation (1). Note fr is a rotating magnetized resonance frequency, μi is an initial permeability, γ is a gyromagnetic constant, and Ms is a saturation magnetization.fr(μi−1)=1γ1×(Ms/3π)  Equation (1)
In addition, Patent Article 1 shows a ratio of the initial permeability at 20° C. and the initial permeability at 85° C., considering temperature change of the permeability in case when a zirconium oxide is added. The smallest ratio of the initial permeability at 20° C. and the initial permeability at 85° C. in examples of Patent Article 1 is 1.45 times. This result is too large to determine that the initial permeability change with respect to temperature change is prevented.
Further, according to Patent Article 2, the initial permeability value is not shown and the frequency characteristic is unknown. According to Patent Article 3, the temperature characteristic is unknown. In addition, according to Patent Articles 1 to 3, their DC superposition characteristics are unknown.
Considering all above, it is unknown that the frequency characteristic, the DC superposition characteristics, and the temperature characteristics of NiCuZn ferrite according to Patent Articles 1 to 3 are all superior.
[Patent Article 1] Japanese Unexamined Patent Publication No. 2000-252112
[Patent Article 2] Japanese Unexamined Patent Publication No. 2006-206347
[Patent Article 3] Japanese Unexamined Patent Publication No. 2008-300548